Spot welding equipment is used to weld steel sheets overlapped with each other. FIG. 21 is a cross-sectional view illustrating the conventional spot welding of steel sheets 50. As shown in FIG. 21, to perform spot welding of steel sheets 50, an overlapped portion of the steel sheets 50 is sandwiched between a pair of electrodes 52, and a force is applied to the electrodes 52 to pressurize the steel sheets 50 in the direction of the arrow as shown in FIG. 21. A large current on the order of kA is then applied to the electrodes 52 while the pressurized state is maintained to melt the compressed part instantaneously by Joule heating, namely by resistance heating, thus forming a melted mass of a given diameter called a nugget 54 (Non-patent Reference 1, for example). The nugget 54 is also called a weld nugget.
FIG. 22 is a detailed cross-sectional view of a weld zone formed by conventional spot welding. As shown in FIG. 22, a spot weld zone 53 includes the weld nugget 54, a heat affected zone 55 surrounding the weld nugget 54, a corona bond region 57 formed on the boundary between the steel sheets 50, 50 in the heat affected zone 55, and a gap 58 which could be generated on the boundary between the heat affected zone 55 and the steel sheets 50, 50. The heat affected zone 55 is also called HAZ. The corona bond region 57 and the gap 58 are also called a pressure welding zone and a sheet separation tip, respectively.
Furthermore, a sputter 56 may be generated in the gap 58. Steel melted by spot welding extrudes from the weld nugget 54 through the heat affected zone 55 to outside to form the sputter 56 in the gap 58 between the overlapped portion of the steel sheets 50, 50, constituting a part of the weld nugget 54. The sputter 56 is also called an expulsion. As a result of generation of the sputter 56, a blowhole, namely a spherical cavity, may be generated within the weld zone 53, or splashed sputter 56 may be attached to the steel sheets in regions other than the spot weld zone 53. Since the generation of the sputter 56 may result in defects in the painting process to be conducted after the spot welding, it is undesirable. However, generation of the sputter 56 is inevitable at present. If a crack is generated in the corona bond region 57 within the heat affected zone 55 in the spot weld zone 53, the strength of a cross joint, which will be described later, will degrade.
By the way, high-strength steel sheets have recently been used in vehicle production lines as raw materials for vehicle bodies to be spot-welded to reduce the weight of vehicles and ensure safety at the same time.
FIG. 23 is a plan view of samples used for a tensile test to examine the spot weld strength of high-strength steel sheets, where (A) is a lap joint, and (B) is a cross joint. In the lap joint sample as shown in FIG. 23(A), the ends of two rectangular steel sheets 50 in the longitudinal direction are overlapped, and these ends are spot welded. In the cross joint sample as shown in FIG. 23(B), two rectangular steel sheets 50 are crossed to form a shape of a cross, and this cross section is spot-welded. The mostly ellipsoidal portion enclosed by a dotted line is a nugget 54 formed by welding, and the forces 56 applied in a cross tensile test are shown by the arrows.
FIG. 24 illustrates the mode of break of the cross tensile test of the spot weld zone. As shown in FIG. 24, the mode of break is classified into (a) a break of planar fracture in the nugget, (b) a break of plug-like fracture in the nugget, (c) a break of plug-like fracture in the heat affected zone, (d) a matrix break, and (e) a complex type of break not shown in the figure. The complex type of break is a combination of (b) to (d) breaks described above. As the position of break moves from (b) to (d), the cross rupture strength increases.